1. Field of the Invention
The present invention relates to a printhead, head cartridge, and printing apparatus. Particularly, the present invention relates to an inkjet printhead having many printing elements, a head cartridge, and a printing apparatus using any of them.
2. Description of the Related Art
A printhead with an array or arrays of printing elements has conventionally been known. On a printhead of this type, several or several tens of driving integrated circuits capable of concurrently driving N printing elements as one block are formed on the same substrate. Print data are aligned and input in correspondence with the respective printing elements, and can be printed on a print medium such as print paper. Printing apparatuses with printheads of this type can print at high densities and high speeds, and thus are widely used as printers in today's business offices, for other paperwork tasks, and for personal use. Even now, printing apparatuses are developed and improved for further cost reduction, higher resolution, and the like.
A printhead mounted in the inkjet printing apparatus (to be referred to as a printing apparatus hereinafter) is configured by arraying, as printing elements, electrothermal transducers (to be also called heaters hereinafter) for generating discharge energy necessary to discharge ink from nozzles. As a known method for this printhead, printing elements are divided into a plurality of blocks, and the blocks are temporarily driven sequentially or distributedly because large power is necessary to drive printing elements.
Especially for a printing element which prints by discharging ink using heat, if one printing element is continuously driven, heat is accumulated, and the print density may change. The printing element is also influenced by heat of an adjacent printing element. If the printing apparatus concurrently drives adjacent printing elements, nozzles are interfered with mutual pressures generated in ink discharge. The pressure interference (crosstalk) may change the print density. Hence, an idle time for dissipating heat or avoiding crosstalk is desirably set after driving the printing element.
To solve this problem, there is known distributed driving of distributedly driving printing elements to be concurrently driven in the array direction of the printing elements. According to this driving method, adjacent printing elements are not concurrently driven. By setting an idle time, the influence of an adjacent printing element can be eliminated.
FIG. 7 is a diagram showing the arrangement of a printhead which performs time-divisional driving.
In a specific example shown in FIG. 7, an enable signal which is input from a terminal 5a to enable driving a printing element is commonly supplied to all printing elements 1. In FIG. 7, reference numeral 3a denotes a terminal to apply a power supply voltage VH to the printing element as a voltage for driving a heater; and 4a, a ground (GND) terminal.
In a conventional printhead shown in FIG. 7, print data and a clock signal are respectively input from terminals 8a and 8b, and the print data is stored in a shift register 8. A latch signal is input from a terminal 7a, and the print data is latched by a latch circuit 7. By aligning print data in correspondence with printing elements, the printing elements of each block can be energized in accordance with the print data for the period of the latch signal.
Further in this arrangement, a block control signal is input from a terminal 6a and supplied to a decoder circuit 6. The decoder circuit 6 generates a block selection signal for selecting a block of four printing elements on the basis of the input block control signal. An AND circuit 5 receives the print signal from the latch circuit 7, the block selection signal from the decoder circuit 6, and the enable signal. When the logical value of these signals is “1”, the AND circuit 5 outputs a driving signal to a driver (transistor) 2, driving a corresponding printing element.
Time-divisional driving can be achieved by sequentially activating a block selection signal and supplying an enable signal from the terminal 5a in correspondence with each block within the period of each latch signal.
The printhead is configured to deal with various kinds of driving control by shortening the rise/fall time of a driving signal pulse so as to realize high-resolution control within the period of a latch signal.
This technique is disclosed in, e.g., the U.S. Pat. No. 6,116,714.
However, when the conventional printhead is to achieve high print speed, high-resolution color printing, and downsizing, the arrangeable wiring width on the printhead substrate becomes narrow, and the number of concurrently driven printing elements increases. Due to these factors, the print current flowing into the wiring causes the following problem.
This problem is a malfunction of a driving control circuit by switching noise occurred due to a great change of an electric current flowing into a wiring when a pulse-like driving signal rises and falls in concurrent driving. Since the driving signal is controlled temporarily at high resolution, as described above, a driver incorporated in the printhead must be turned on/off quickly. Assuming that the rise/fall time t of the driver is 100 nsec, the self-inductance L of the wiring is 100 nH, and the current I flowing at this time is 1 A, an induced voltage V generated at this time is given byV=L·dI/dt=100×10−9×1/100×10−9=1 V
From this, the induced voltage as high as 1 V is generated as noise.
This noise level greatly affects a logic gate circuit formed from a CMOS, TTL, or the like. Especially for a CMOS circuit whose logic voltage is 3.3 V or less, this induced voltage value almost reaches the threshold level. The switching noise may cause a fatal influence on the printhead operation on a head substrate prepared by integrating, on the same substrate, a printing element driver for switching a large current, and a logic gate circuit formed from a CMOS, TTL, or the like.
The print speed and print resolution are increased by increasing the number of printing elements of the printhead. As the number of printing elements increases, the number of time-divisionally driven blocks and the number of concurrently drivable printing elements may also increase. However, in view of increasing the print speed, the increase of the number of blocks is restricted. This naturally leads to increasing the number of concurrently drivable printing elements. This means that the instantaneous change of the current value becomes large and the noise level becomes high.
The problem of switching noise has conventionally been known, and several countermeasures against this problem have been proposed.
For example, input of a driving signal pulse to printing elements to be concurrently driven is delayed stepwise. According to this method, considering the level and occurrence time of switching noise, delay elements are properly inserted into driving signal lines to delay stepwise, by more than the occurrence time, a timing when the driving signal pulse is applied. This method can suppress occurrence of switching noise. However, according to this method, if the number of concurrently driven printing elements increases, the total delay time becomes long. This results in causing restriction on assigning the driving signal pulse width permissible time during which all printing elements are driven within the printhead printing period (i.e., time for giving a chance to drive all printing elements).
As another method, the rise time of the driving signal pulse and the instantaneous current value are specified, and wiring lines and terminals are dielectrically isolated to adjust the print current to the specified value or less. Even according to this method, the increase in print current by the increase in the number of concurrently driven printing elements cannot be satisfactorily coped with by the dielectric isolation of wiring lines and terminals.